1. Field of the Invention
The present invention relates to an exposure apparatus, and more particularly an exposure apparatus provided with a light source emitting light of ultraviolet region, such as an excimer laser, a harmonic wave laser or a mercury lamp.
2. Related Background Art
Recent advancement of the semiconductor integrated circuits in the level of integration is requiring submicron formation of the minimum circuit geometry. One of the means for achieving such miniaturization is the reduction of wavelength of the light source for the projection exposure apparatus, employed for the preparation of the semiconductor integrated circuits. Currently conceived candidates of such short wavelength light source for the projection exposure apparatus are, for example, a wavelength of 248 nm of the KrF excimer laser, 193 nm of ArF excimer laser or a harmonic wave of Ti-sapphire laser, 266 nm of the 4-times harmonic wave of YAG laser and 213 nm of the 5-times harmonic wave of YAG laser.
FIG. 11 is a schematic plan view of such laser exposure apparatus. The light from an unrepresented light source illuminates a mask of a reticle R formed with a circuit pattern through an illuminating system unit 4, thereby transferring said circuit pattern onto a photosensitive substrate (wafer). Said mask and wafer are positioned respectively on two stages provided in a chamber 6, in which an air conditioning system 7 is provided for maintaining constant ambient conditions in said chamber. For this purpose the air conditioning system 7 circulates air, of which temperature is controlled by a temperature regulator 8, within the system at a constant velocity by a fan 9. At the air inlet to the main body 6a, there is provided a HEPA (high efficiency particulate air) filter 10 for preventing intrusion of particles into the main body 6a, thereby maintaining the cleanness therein at a certain level.
The formation of semiconductor patterns with exposure apparatus utilizing a light source of a long wavelength such as of g- or i-ray has been conducted by an already established process of exposure and development of so-called novolac type photoresist (photosensitive resin) consisting of a novolac resin and a sensitizer. However, when the light source wavelength is reduced for example to 248 nm of the excimer laser, such novolac type photoresists cannot provide a pattern of satisfactory profile because of the increased light absorption of the resin. Consequently, for the exposure apparatus utilizing the light of shorter wavelength such as of the excimer laser, there has been introduced a new family of photoresists called chemical amplification type, which has become predominant at present, because of the superior performances such as pattern profile, resolving power and sensitivity. The chemical amplification photoresist is generally composed of a resin, a photosensitive acid generator and a dissolution accelerator or a crosslinking agent. The acid generator generates an acid at the exposure, then said acid functions as a catalyst at the postexposure bake (PEB) to accelerate the reaction of the dissolution accelerator or the crosslinking agent, and the pattern is formed at the development. A positive- or negative-working pattern is formed respectively by the use of the dissolution accelerator or the crosslinking agent.
The above-mentioned apparatus utilizes various optical system, including the illuminating system 4, lenses (such as relay lenses and a projection lens PL) and mirrors, and the optical materials constituting such optical systems become hazy by the irradiation of the light of ultraviolet region, thus losing transmittance. For this reason the conventional apparatus has been associated with a drawback of a decrease of the ultraviolet irradiation reaching the wafer surface.
Such gradual decrease of transmittance of the optical materials has conventionally been coped with by the cleaning or replacement of the smeared optical materials. However, such cleaning or replacement inevitably involves a movement of the optical system, and necessitates therefore the adjustment of the optical axis etc. During such adjustment the apparatus is not operable, so that a long down time is therefore unavoidable.
Also the chemical amplification photoresists, employed for such ultraviolet exposure apparatus, are superior in the resolving power and the sensitivity, but lack stability, because of the difficulty in the control of acid generation by exposure and of acid catalyst function by postexposure bake. Particularly in the positive working photoresists, if basic gas such as amine is present in the atmosphere from the exposure to the postexposure bake, the acid generated by the exposure in the vicinity of the photoresist surface escapes into the air by reaction with such basic gas, thus causing surface insolubilizing phenomenon, in which the exposed area, to be dissolved in the developer, becomes insoluble. FIG. 4A shows a rectangular photoresist profile, obtained with novolac type photoresist, and with an exposure apparatus utilizing an exposure wavelength of g- or i-ray. If a similar pattern is formed with a positive-working chemical amplification photoresist, the obtained profile becomes T-shaped with a cusp at the top as shown in FIG. 4B, if such insolubilized layer is formed. Such T-shaped profile significantly hinders the succeeding steps such as etching, rendering faithful reproduction of the pattern impossible. Such basic gas is often present in the air of the clean room, thus hindering the use of the chemical amplification photoresists.
Also the conventional apparatus is incapable of eliminating the cause of smear, present in the apparatus.